fix changelog title levels

CHANGELOG.md

@@ -18,12 +18,12 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Added MIME type for WEBP image format (#246)
 - Added more validation of values according to PSR-7 and RFC standards, e.g. status code range (#250, #272)
 
-## Changed
+### Changed
 
 - Tests don't pass with HHVM 4.0, so HHVM support got dropped. Other libraries like composer have done the same. (#262)
 - Accept port number 0 to be valid (#270)
 
-## Fixed
+### Fixed
 
 - Fixed subsequent reads from `php://input` in ServerRequest (#247)
 - Fixed readable/writable detection for certain stream modes (#248)